X-linked

Report
Sex-Linked Traits
&
Pedigrees
Linked Genes
• Some genes are linked
because they are on the
SAME chromosome.
• A chromosome is a set of
linked genes.
• Remember: Offspring get one
copy of each chromosome
from each parent.
• Chromosomes assort
independently, NOT individual
genes.
• Crossing-over during meiosis
can “un-link” genes.
Autosomal vs. Sex-Linked
Traits can be either:
• Autosomal: traits (genes)
are located on the non-sex
chromosomes
• Sex-Linked: traits (genes)
are located on the sex
chromosomes
• Sex chromosomes
determine gender (X & Y)
• XX genotype for females
• XY genotype for males
• Many sex-linked traits are
carried on X chromosome
Human Karyotype
Picture of Human Chromosomes
22 Autosomes and 2 Sex Chromosomes
Examples of
Autosomal Traits
Autosomal Dominant:
Huntington’s Disease
Achondroplasia (Dwarfism)
Osteoporosis
Autosomal Recessive:
Cystic Fibrosis
Sickle Cell Anemia
Examples of
Sex-Linked Traits
X-linked Recessive:
Colorblindness
Hemophilia
Duchenne Muscular
Dystrophy
Y-linked:
Ear Hair
What this means…
• Males have only one X
chromosome, so ALL X-linked
alleles are expressed in
males, even if the trait is
recessive.
• This is why males exhibit
some traits more frequently
than females.
• X-linked traits are passed
from mother to son.
• Y-linked traits are passed
from father to son.
Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
XY chromosome - male
copyright cmassengale
10
Sex-linked Trait Problem
• Example: Eye color in fruit flies
• (red-eyed male) x (white-eyed female)
XRY
x
XrXr
• Remember: the Y chromosome in males
does not carry an allele for the trait.
• RR = red eyed
Xr
Xr
• Rr = red eyed
• rr = white eyed
XR
• XY = male
• XX = female
Y
copyright cmassengale
11
Sex-linked Trait Solution:
Xr
XR
XR
Xr
Y
Xr Y
Xr
XR
Xr
Xr Y
50% red eyed
female
50% white eyed
male
copyright cmassengale
12
Pedigrees
A tool that can be used to trace
genetic trait or disease throughout
several generations.
Show ACTUAL results.
Carriers
• Heterozygote (has the
recessive allele but the
trait is NOT expressed)
• Shown in a Pedigree by
shading half of the circle
(female) or square (male)
• In autosomal traits,
carriers can be Male or
Female.
• In Sex-Linked traits, ONLY
Females can be carriers. All
Sex-Linked traits are
expressed in Males.
Sex-Linked Pedigrees
copyright cmassengale
15
Tips for Solving Pedigrees
You must figure out 2 things in
order to correctly solve a
problem involving a Pedigree:
1. Determine if trait is
Dominant or Recessive
2. Determine if trait is
Autosomal or Sex-Linked
Dominant Alleles
• Appears EVERY generation
• Expressed even in heterozygotes
• Will appear to be relatively
common in a pedigree
• A dominant trait will not occur in
an individual unless it also appears
in at least one of the parents.
• Unaffected parents will have only
unaffected offspring
Recessive Alleles
• A recessive trait commonly skips
one or more generations
• Frequently found in pedigrees that
include marriage between close
relatives
• If BOTH Parents are affected, ALL
of the offspring will be affected
• If both parents are heterozygous
(dominant allele is expressed),
each offspring:
– 50% chance of being
heterozygous (carrier)
– 25% chance of being homozygous
recessive (affected)
Autosomal Traits
• If there is a male Heterozygote
(carrier), the trait MUST be
autosomal.
• An autosomal trait can be passed
on from father to son OR daughter
• Approximately the same number
of males and females will be
affected (especially if autosomal
recessive)
Sex-Linked Traits
• Must determine if the trait is
X-linked or Y-linked
• An X-linked trait can NEVER be
passed from father to son
– Any example of father to son
transmission means that the trait is
NOT X-linked
• Y-linked traits will NEVER affect
females
– Any example of a female with
the trait means that the trait is
NOT Y-linked
Y-Linked Traits
• No affected females
• ALL sons of affected males are
also affected
X-Linked Recessive Traits
• Both males and females can be
affected, but the trait is much
more common in males
• Affected males transmit the allele
to ALL of their daughters & NONE
of their sons
• Daughters of affected males are
always be carriers (not affected)
unless mother also has the allele
• All sons of an affected female
will also be affected
X-Linked Recessive
X-Linked Dominant Traits
• If the trait is dominant, it will be
expected to occur slightly more
often in females
• ALL daughters of an affected male
will be affected (NO carriers)
• Can not be passed on from father
to son

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